The present invention relates to the field of echo cancelers generally, and more specifically to acoustic echo cancelers
A call to a wireless mobile telephone passes through a base station with which the mobile unit is in contact. The speech signals transmitted from the base station of the wireless system may be transmitted from the earpiece or loudspeaker of the mobile unit to the mouthpiece or microphone of the mobile unit. This coupling may result from direct propagation of the sound through the air and/or coupling of the sound through the material of the mobile unit (i.e., the handset) itself. The transmitted mobile signal arrives back at the base station with an amplitude and a time delay that are functions of the medium in which the sound wave propagates, such as the speed of sound, and the distance of the propagation path.
Back at the base station, the acoustically coupled or directly propagated sound adds with the speech from the mobile unit. The coupled sound may have a small effect if the delay time is very short. A long delay produces a distracting, distinct repetition of the caller to the mobile""s original speech (an echo). Whether the delay is long or short, the echo produced by such coupling is undesirable.
Echo cancelers are known for use in systems that are prone to echoes, and operate by anticipating the echo that a signal will produce and subtracting the anticipated echo from the signal. The main elements of an echo canceler include an adaptive filter and a subtracter. The adaptive filter correlates a reference signal (presumably a duplicate of the caller""s voice signal) with the voice signal returning along the echo path. Over a short period of time, numerical factors are developed that model the echo path. The factors define the impulse response of the echo transmission pathway, which embodies aspects of attenuation and time delay. The reference signal is then multiplied by the impulse response as represented by the factors, producing an echo replica signal that is subtracted from the actual echo signal received along the echo path.
The difference or error is fed back to the adaptive filter. The adaptive filter adjusts its coefficients (or taps) as necessary to minimize the error. The coefficients are initially calculated over a short period at the beginning of a call, and thereafter are refined. Normally, reasonably accurate coefficients are reached promptly. The echo path is typically due to the character and fixed geometry of the signal transmission path. Normally the signal path does not change, and after the coefficients are reached they do not change substantially for the duration of a connection. However it is possible that the transmission path could change (for example, in the case of a hands-free speaker phone in an enclosure with doors or windows that can be opened). It is not possible to predict the echo response, and for these reasons, the echo canceler continuously updates or revises the coefficients, and adaptively cancels the echo by adjusting the coefficients.
FIG. 1, labeled prior art, shows a typical arrangement in the form of a cordless telephone system 9 that has a base station 9a and a mobile unit 9b. A sound signal 20 is transmitted from base unit 9a via antenna 21, shown as wireless signal 22, to a receiving antenna 23 of mobile unit 9b. The signal typically contains a voice signal. There is a coupling path 24, for example due to acoustic coupling between the speaker and microphone of the mobile unit 9b. Mobile unit 9b transmits the echo signal as a wireless signal 26, from antenna 25 of the mobile unit 9b to antenna 27 of the base station 9a. The reference signal 20xe2x80x2 (which is a duplicate of the original sound signal 20) and the returning signal 28 are fed to the adaptive filter 29, which outputs a replica signal 29a. The received replica signal 29a is subtracted from the echo signal 28 in subtractor 30, to form a feedback signal 31 that is fed back to the adaptive filter 29.
The echo canceler in the device shown in FIG. 1 is associated with the device that produces the echo. Therefore, the device operates substantially for the benefit of the remote correspondent by eliminating the echo produced locally at the mobile unit.
In a simple theoretical arrangement wherein there is a linear relationship between the reference signal and the echo, the returning echo signal might be characterized as a delayed copy of the sound signal and the reference signal, having an amplitude that is proportionately reduced relative to the reference signal. In that case the impulse response of the echo path simply represents a delay. If there are no non-linear operations performed on the sound signal, it is possible to delay the reference signal in the adaptive filter, so as to have the same phase as the echo. The echo response, however, may be more complicated than that.
In a typical cordless telephone system, signals are compressed (encoded) from samples at 64 kilobits per second to eight kilobits per second before being transmitted between the base station and the mobile unit. The compressed data is decompressed (decoded) by the recipient. FIG. 2 shows the effect of adding encoding and decoding stages into the signal processing path. In FIG. 2, a prior art cordless telephone system 10 includes a base station 10a and a mobile unit 10b. A sound signal 40 is encoded at encoder 41. The encoded signal 42 is transmitted from base unit 10a via antenna 43, as wireless signal 44, to a receiving antenna 45 of mobile unit 10b. The signal is decoded in decoder 46 and provided to a speaker (not shown). There is an acoustic coupling 47 between the speaker and microphone (not shown) of the mobile unit 10b, providing an echo transmission path. The input to the microphone is encoded in encoder 48. Mobile unit 10b transmits the encoded signal 49 as a wireless signal 51, from antenna 50 of the mobile unit 10b to antenna 52 of the base station 10a. The signal is decoded at decoder 53 of the base station 10a. The reference signal 40xe2x80x2 (which is a duplicate of the original sound signal 40) is also fed to the adaptive filter 55, which outputs a replica signal 55a. The received replica signal 55a is subtracted from the echo signal 54 in subtractor 56, to form a feedback signal 58 that is fed back to the adaptive filter 55.
The encoders 41, 48 and decoders 46, 53 perform non-linear operations. The coupled earpiece output is added prior to the second non-linear encoding operation and the second non-linear decoding operation. The speech signals 44 transmitted from the base station to the mobile unit undergo a round of encoding and decoding on the downlink, and the acoustic echo in the mobile unit 10b undergoes a second round of encoding and decoding in transmission over the uplink. The acoustic echo in the sound signal 51 transmitted to the base station is thus subjected to a non-linear transformation with respect to the reference signal 40xe2x80x2. In short, there is no precise delay time associated with the echo transmission path, as is characteristic of a simple acoustic echo transmission path. Standard echo-cancellation techniques, which assume a linear, coherent relationship between the reference signal and the echo, at least at a particular frequency, are not effective.
An improved echo canceler is desired to accommodate this complication.
The present invention is a method and apparatus for echo cancellation in a system having first and second units in communication with each other, the method including a plurality of encoding and decoding operations performed on a reference signal to form a modified reference signal, the plurality of encoding and decoding operations matching a set of encoding and decoding operations performed on a sound signal by the first unit and the second unit.
In a method according to the invention, a reference signal matches a sound signal. A plurality of encoding and decoding operations are performed on a reference signal to form a modified reference signal. The plurality of encoding and decoding operations match a set of encoding and decoding operations performed on the sound signal by the first unit and the second unit. The modified reference signal is applied to an adaptive filter to form a replica signal. The replica signal is subtracted from an echo signal received by the first unit.
Another aspect of the invention is an acoustic echo canceler for a system having a first unit and a second unit in communication with each other for transmitting a sound signal. A plurality of encoders and decoders receive a reference signal that matches the sound signal, and form a modified reference signal. The plurality of encoders and decoders match a set of encoders and decoders that operate on the sound signal in the first unit and the second unit. An adaptive filter receives the modified reference signal and forms a replica signal. A subtractor subtracts the replica signal from an echo signal received by the first unit.